The increases in the machining speed in the field of machine tools have led to problems such as a lowering of the machining accuracy due to the vibration of the electric drive motors used for driving the spindle and other movable components of a machine tool, and an increase of the noise therefrom. It is considered that the vibration of an electric motor can be most effectively reduced by correcting an unbalanced state of each rotating component of the electric motor.
Therefore, in the conventional procedure for assembling an electric motor, the respective rotating components of the motor are first balanced to thereby obtain a well-balanced final product as the electric motor.
In a very-high speed motor operating at a maximum rotational speed of more than 15,000 r.p.m., however, even a very small imbalance therein can cause detrimental vibration and noise when the motor operates in the high-speed range. Namely, if the rotating components of an electric motor are balanced individually and the components are then assembled, very small imbalances remaining in the components may accumulate to form an imbalance that exceeds an allowable limit.
Even if it is intended to complete the construction of the motor by pre-assembling the rotating components thereof and balancing same before combining the balanced rotating components with a motor casing and associated parts, it is frequently impossible to carry out such an assembling process due to structural restrictions, and even if it is possible to assemble the motor by such assembling method, a problem arises in the quality control of the motor. For example, it is necessary to take the bearings to pieces, to attach only the inner rings to the output shaft of the motor, and to reassemble the bearings after balancing the combination of the output shaft and the inner rings. Such a method requires much time and labor, and unavoidably gives rise to problems such that the bearings are contaminated with dust. Furthermore, the parts and components of the motor must be mounted on an external drive means of a commercial balancing machine, to be dynamically balanced individually, but since the external drive means of the commercial balancing machine can rotate the parts only at 3,000 r.p.m. at most, the commercial balancing machine is incapable of detecting a small imbalance thereof.